The present invention relates to a method for tuning the channel of broadcasting waves, and more particularly relates to a method for tuning the channel of broadcasting waves received by a television set capable of receiving broadcasting waves of CATV stations.
Generally, in referring to the signal wave equivalent to the broadcasting wave of a CATV station, each channel is allocated to plural band zones which are predetermined. And, each channel is also assigned to a different frequency zone of channel plan according to the region.
For example, in the United States either the Normal channel plan or the HRC channel plan having different frequency zones is allotted to each channel, as shown in Table 1.
TABLE 1 __________________________________________________________________________ Sound IF 41.25 MHz Picture IF 45.75 MHz TV/CATV CHANNEL PLAN NORMAL HRC CHNL TV fp fosc CATV fp fosc fp fosc IND CHNL (MHz) (MHz) CHNL (MHz) (MHz) (MHz) (MHz) __________________________________________________________________________ 1 -- -- -- 5A 73.25 119.00 72.00 117.75 2 2 55.25 101 2 55.25 101.00 54.00 99.75 3 3 61.25 107 3 61.25 107.00 60.00 105.75 4 4 67.25 113 4 67.25 113.00 66.00 111.75 5 5 77.25 123 5 77.25 123.00 78.00 123.75 6 6 83.25 129 6 83.25 129.00 84.00 129.75 7 7 175.25 221 7 175.25 221.00 174.00 219.75 8 8 181.25 227 8 181.25 227.00 180.00 225.75 9 9 187.25 233 9 187.25 233.00 186.00 231.75 10 10 193.25 239 10 193.25 239.00 192.00 237.75 11 11 199.25 245 11 199.25 245.00 198.00 243.75 12 12 205.25 251 12 205.25 251.00 204.00 249.75 13 13 211.25 257 13 211.25 257.00 210.00 255.75 . . . . . . . . . . . . . . . . . . . . . . . . . . . __________________________________________________________________________
In this Figure, for example, each CATV channel corresponding to channels 1 to 13 is constructed with either one of Normal channel plan or HRC channel plan. With respect to each of the center frequencies thereof, only channel 5 and channel 6 in the Normal channels are deviated as much as 0.75 MHz towards lower frequencies than that of the HRC channels, and the center frequencies of the Normal channels for all of the other channels are deviated as much as 1.25 MHz towards higher frequencies than that of the HRC channels.
Some television receivers capable of receiving CATV signals and television signals are provided with a tuning function for tuning the phase-locked loop (hereafter referred to as PLL tuning). A block diagram of such television receiver 30 is shown in FIG. 3.
In this Figure, the numeral 31 is a microcomputer controlling the whole system (hereafter referred to as CPU), and the numeral 32 is a PLL tuner portion tuned to the received channel, and the numeral 33 is a memory which memorizes the received channel number. The PLL tuner portion 32 is provided with a tuner/IF portion 321 and a PLL circuit 322.
The PLL circuit 322 is provided with a band switch 325, an oscillator portion 326 outputting the standard oscillating frequency signal having 4 MHz frequency, a phase comparator 327, a low pass filter 328, a prescaler 323 and a frequency divider 324. The tuner/IF portion 321 is provided with a VCO 321a functioning as the tuner portion receiving the selected channel and as one part of the above-mentioned PLL circuit 322, and a intermediate frequency amplifying portion 321b. The numeral 34 is an antenna which receives the television waves transmitted from the broadcasting stations, and numeral 35 is a cable for the CATV. The numeral 36 is a switch switching the received signal from the television antenna 34 and the received signal from the CATV cable 35. Also, the numeral 37 is an image signal conditioning circuit and the numeral 38 is a cathode ray tube.
In FIG. 3, the memory 33 in the television receiver 30 stores each channel's initial data under the Normal channel plan (namely, center frequency, the dividing ratios and others). In selecting a particular channel, the CPU 31 equivalent to the system controller of the television receiver 30 reads out the stored data of the channel, converts the data and transmits the data into the PLL circuit 322.
As the channel data are read out in accordance with the Normal channel plan, the PLL circuit 322 performs phase synchronization (PLL) by applying the frequency according to the data of the aforementioned Normal channel plan, and lets the tuner/IF portion 321 accommodate the specified channel frequency.
Wherein, the band zones as the PLL tuner portion 32 are formed in between 2 MHz towards the upper and lower sides from the tuned center frequency. Accordingly, for example, when channel 3 is designated, the receivable range lies in between 59.25-63.25 MHz equivalent to the frequency adjusted as much as 2 MHz towards the upper and lower sides from the center frequency of 61.25 MHz.
However, when channel No. 3 in this region belongs to the Normal channel plan, it becomes possible to receive channel no. 3 as it is without any problem, but when channel no. 3 in this region belongs to the HRC (Harmonically Related Carriers) channel plan, the 58-62 MHz adjusted as much as 2 MHz towards the upper and lower sides in centering with the 61.25 MHz corresponding to the tuned frequency of Normal channel no. 3, become the required band zone for receiving. But as this receiver is tuned with the Normal channel, then the receivable band zone is 59.25-63.25 MHz as mentioned above, and the lower side of the frequency zones is cut off as much as 1.25 MHz. This cut-off zone is shown as Mark B in FIG. 2. Although it is, of course, possible to receive TV signals in the cut-off zone, the quality of received images is unavoidably deteriorated, because the receivable band zone is substantially restricted.
Further, as the tuned frequency differs in the Normal channel plan and the HRC channel, it requires surplus time to tune the PLL tuning starting from a tuning frequency to the other tuning frequency.
The present invention is developed to overcome the above-mentioned drawbacks in the conventional prior art, and its object is to provide a method for tuning the channel of broadcasting waves which is able to automatically obtain the required band zone and to shorten time required for tuning.